SEPARATOR FOR REMOVING IMPURITIES FROM A FLUID

Abstract: The invention relates to an apparatus for removing impurities from a fluid, in particular to a hydrocyclone apparatus having a lower particle trap, the apparatus comprising a tank (10) with a sidewall (12) of substantially circular cross-section and an inlet (11) for introducing fluid into the tank, the inlet of flow being such that it is caused to circulate around the tank (10). An outlet (13) is arranged for conducting fluid, with impurities removed, out of the tank (10). A wall/screen means (15) is arranged in the tank and extends from a location (16) on the sidewall (12) in the direction of fluid circulation and across the outlet (13). The wall/screen means (15) is arranged at a distance from the sidewall such that fluid leaving the tank (10) will leave in the opposite direction of flow to that of the circulating fluid and will thus be relatively free of impurities.

Description: Title: Separator for removing impurities from a fluid Field of the Invention The invention relates to an apparatus for removing impurities from a fluid, comprising a tank having a sidewall with a substantially circular cross-section, an inlet arranged in said sidewall for introducing said fluid into said tank in a manner so as to cause said fluid to circulate in said tank, and an outlet arranged for conducting fluid out of said tank. In particular, the invention relates to a hydrocyclone used for removing solid particles such as sand, from an inlet fluid such as the digesting, washing or bleaching liquors used in the production of cellulosic pulp. Background art A device of the aforementioned type is known for example from US-A-4 280 902. In this device, the fluid containing the solid impurities (in particular sand) is pumped into the separation tank through a substantially tangential inlet with respect to said tank, thus causing the fluid to circulate or whirl around the interior of the tank. Since the circulating force of the fluid is quite high, the solid particles in the fluid tend towards the sidewall of the tank due to the centrifugal force. Upon reaching the sidewall and on their way to the sidewall, the particles sink downwards to the lower part of the tank and into a suitably arranged sand trap, which can be periodically emptied as required. At the opposite end of the tank to the sand trap, i.e. the upper wall, a main outlet for the relatively purified fluid is arranged. Since the sand or other heavier impurities will be travelling outwardly and downwardly, the fluid which is at the upper part of the tank will be relatively free of solid particles and will exit through the aforementioned outlet. For further details of hydrocyclone operation and suitable sand trap arrangements, including stagnation avoidance devices which may be included with the device of the present invention, the content of US-A-4 280 902 is hereby incorporated by reference. In one particular application of a separation device, the outlet fluid from the separation tank is fed to the chip chute of a pulping plant, said chip chute being arranged between a relatively low pressure (e.g. 2 bar) device, such as a steaming vessel for wood chips, and a high pressure feeder (HP-feeder) responsible for pumping chips to an impregnation vessel (or the digester, if a one-vessel system is used) , said chip chute carrying moisturised chips. The fluid, including fluid coming from the chip chute (and also fluid from the impregnation vessel) , which is not transported with the chips from the high pressure feeder is then reintroduced into the separation device for further separation (since it now contains further impurities which were present with the chips coming from the steaming vessel) . In order to be able to provide for storage of excess fluid (e.g. wood moisture and HP-feeder lubricating liquid in addition to the normal liquors) leaving the separation tank, i.e. in the case where the flow is greater than needed, a buffer tank is required. Fluid is fed to the buffer tank from the separation tank, in the prior art devices, by a branch connection from the main outlet or by a branch connection downstream thereof. Some of the buffer tank fluid is used downstream of the HP- feeder e.g. in the impregnation vessel. Accordingly it must be pumped from the buffer tank to a very high level/pressure which has to be formed by means of a pump having extremely high head (e.g. having a pump pressure, p pump . greater than 100 mwc and preferably greater than 150 mwc - where "mwc" stands for "metre water column") , preferably a centrifugal pump having a closed impeller wheel being used. Hence the fluid must contain no particulate material which would otherwise very soon damage the expensive pumping arrangements. Whilst such particle separation has been fairly effective, some particles can still exit through the fluid outlet together with the fluid, especially less dense particles such as wood chips for example, since the influence of gravity on these is of course less than on the denser particles. Since however the presence of these particles is not acceptable for pumping, as explained above, they must be removed and this has been accomplished in prior art devices by means of one or more so-called in-line drainer(s) which is/are normally connected upstream of, and in line with, the fluid inlet of the buffer tank. This extra filtering or separation section is however costly and also adds to the maintenance routines that have to be carried out with respect to this additional section. An object of the invention is thus to reduce cost by providing an apparatus which removes impurities from a fluid outlet of the separation tank and thus obviates the need for any extra in-line drainer or other additional external separator unit for the buffer tank fluid. Summary of the invention The aforementioned object and further objects are achieved by a device in accordance with the invention having the features defined in claim 1. Further preferred features of the invention are defined in the dependent claims. Brief description of the drawings The invention will now be described with reference to a preferred embodiment thereof as depicted in the following drawings, in which: Fig.l depicts a view of an apparatus made in accordance with the invention, and Fig.2 shows a plan view of the device of Fig.l. Detailed description of a preferred embodiment Fig.l depicts a view of one embodiment of the separation apparatus in accordance with the present invention. A metal tank 10 comprising four main parts, namely a top section 27, a substantially circularly-cylindrical sidewall section 12, a conical sidewall section 21 and a sand trap section 23 are attached to each other to form an enclosed space. An inlet orifice 11 is provided in the sidewall section 12 at approximately the mid-portion thereof and has attached thereto, or integral therewith, an inlet pipe 25, through which is fed the fluid containing dense particles to be removed, for example emanating from an outlet of the high pressure feeder in a pulping plant. The fluid is fed by suitable means into the tank 10 so that it enters the tank with a force causing the fluid to circulate around the tank. In the steady state the tank will of course be full and the body of fluid will all be rotating. The inlet pipe 25 is so arranged, in its preferred form, to introduce fluid into the tank in a substantially tangential manner as shown. The direction of rotation of the fluid in the tank will therefore be as shown by the arrow in Fig. 2 and the corresponding arrow in Fig.l. Due to the fact that fluid is circulating, centrifugal force on the dense particles contained therein will cause the particles to move through the volume of the fluid to the sidewalls of the tank. The denser particles, due to the effect of gravity, move downwardly towards the sand trap section 23 of the device, which will not be described in detail here, where the particles are held. Periodically the sand or other particles are removed via the arrangement 24 which forms a sluice valve set-up. A vertical stagnation prevention device 22, not shown in detail, may also be present in the sand trap section and may correspond to the sand trap in the aforementioned US-A-4 280 902. The tank thus operates as a hydrocyclone separator. The fluid at the upper part of the tank will exit via the main outlet 17 and the outlet 13 to a buffer tank (not shown) by way of outlet pipe 26 which is arranged substantially tangentially with respect to the tank in a manner similar to the inlet pipe, although in the opposite direction with respect to fluid circulation. Normally the outlet through the main outlet 17 is much greater than that through the outlet 13 to the buffer tank. Unless additional measures are taken however, the circulating fluid at the upper part of the tank may still contain some particles (especially less dense particles, such as wood chips) which will exit via outlet 13, requiring their subsequent removal external to the tank. In order to increase the separation efficiency or, in other words, to provide improved separation of particulate material from the fluid at this location so that no (or very little) particulate material exits from the tank via outlet 13 to the buffer tank, a wall/screen means 15 is provided to prevent particulate material exiting the tank via the outlet 13. The structure and function of the wall/screen means will be explained in the following. The wall/screen means in the preferred embodiment comprises a plate-like element which is attached to the sidewall at location 16 in such a way that no circulating fluid, or at least no substantial part of it, may pass between it and the sidewall at the level of the outlet 13. A suitable means of attaching the wall/screen means 15 would be by welding for example. As can be seen more clearly in Fig.2, the wall/screen means 15 extends from location 16 around preferably about one quarter of the periphery of the tank and thus extends upstream and downstream of the outlet 13 with respect to the fluid circulation direction. The wall/screen means is preferably so arranged that a gap is formed between it and the tank sidewall 12, being substantially totally open at the downstream end thereof and separated from the sidewall by a distance x 2 . Depending on the required structural rigidity of the wall/screen means, suitable attachment means between the wall/screen means 15 and the tank at locations other than just location 16 will be provided. The wall/screen means 15 is shown as having an ovally arcuate shape in plan view, extending in the direction of fluid circulation at an increasing distance x,, x 2 from the sidewall 12. In this way, the fluid circulation path around the tank undergoes very little interruption upon meeting the wall/screen means 15 since its direction is modified slowly and smoothly. The invention is however clearly not limited to such an embodiment and the plate could for example be formed by a short leading section at location 16 or thereabouts and then a plate having a circularly arcuate shape which is at a constant distance from the sidewall. Similarly, the wall/screen means need not extend as far as shown with respect to the outlet if the circulation velocity of the fluid is sufficient for efficient operation, or the wall/screen means may extend further if deemed appropriate. Furthermore, it is evident for the skilled man that the optimal design of the wall/screen means 15 would differ depending on the circumstances, e.g. fluid velocity etc. Accordingly in some separators a non-penetrable wall is preferred whereas in others a partly or totally penetrable (e.g. with punched holes) wall/screen means may be preferred. This may the case especially where excessive turbulence at the rear edge of the wall/screen 15 means. The wall/screen means operates in the following manner. Each of the particles still present in the circulating fluid (e.g. as a result of vortices or other factors) which is to be removed, is generally denser than the fluid itself and will thus have a large component of velocity and momentum in the direction of circulation. Such particles which reach the height of the outlet 13 will thus be constantly entrained around the tank and will therefore be directed along, or close to, the wall/screen means 15. Fluid which exits through outlet 13 is relatively slow- moving behind wall/screen means 15 and travels in the opposite direction to the main circulation. Due to the difference in densities between the fluid and the particles, the particles will in the main not be entrained with the fluid towards the outlet 13. Since the wall/screen means is open towards the bottom of the tank, those few particles that are drawn into the fluid behind the wall/screen means will fall under the force of gravity down below the wall/screen means again and will not exit via outlet 13 since the fluid is relatively slow-moving towards the outlet, allowing time for the particles to descend. Normally, the speed of flow towards outlet 13 varies in dependence on the distance of the wall/screen means 15 from the tank wall 12, such that the smaller the distance is, the smaller the velocity is. The distance must thus be determined according to circumstances when constructing the tank. Particles of less density, such as certain wood chips, will not be affected by gravity and will remain in a floating state. Accordingly such particles will follow the upward circulation flow in the tank. Due to the wall/screen means however they cannot escape via outlet 13, but will eventually escape via the main outlet 17. From the main outlet these woodchips etc. will follow the liquid stream back to the chip chute, i.e. without causing any problems of reintroduction. As shown in the figures, the wall/screen means has a vertical extension both above and below the outlet 13, which should be of a sufficient height to prevent vortex and/or other effects at the upper and lower 19 edge portions of said wall/screen means from drawing in solid particles from the main circulation into the reverse flow behind the wall/screen means. The fluid inlet 11 is preferably arranged below the outlet 13 and the wall/screen means 15. In this manner, a large proportion of the solid particles in the inlet fluid will already have been separated out due to the combined centrifugal and gravitational forces before reaching the height of the outlet 13. The "main" tank outlet 17, which for example is connected to the chip chute in a pulping plant, extends down to approximately the same level as the inlet 11, its lower edge 20 being below the lower edge 19 of the wall/screen means. This outlet is arranged substantially centrally and, due to its central location, will be relatively free of dense particles since the centrifugal force will generally have caused these to migrate to the tank sidewall. The embodiment described above is given by way of example only and should not be regarded as limiting for the invention which can be varied widely within the scope of the appended claims. By way of example it will be understood that the wall/screen means may be designed in different manners comprising different patterns, shapes or widths etc. Moreover it would be possible to use more than one wall/screen means, e.g. a combination of two or more different wall/screen means faces in overlapping relationship with each other.

Claims: Claims 1. Apparatus for removing impurities from a fluid, comprising a tank (10) having a sidewall (12) with a substantially circular cross-section, an inlet (11) arranged in said sidewall for introducing said fluid into said tank in a manner so as to cause said fluid to circulate in said tank (10) , and an outlet (13) arranged for conducting fluid out of said tank (10), characterized in that a wall/screen means (15) is arranged within said tank, said wall/screen means (15) extending from a location (16) at said sidewall (12), in the general direction of fluid circulation and across said outlet (13) . 2. Apparatus according to claim 1 characterized in that said wall/screen means is spaced from said sidewall, at least in the region adjacent said outlet (13) . 3. Apparatus according to claim 2, characterized in that the wall/screen means (15) comprises a curved plate attached to said sidewall at one end (16) and extending in the direction of fluid circulation at an increasing distance (x,, x 2 ) from the sidewall (12) . 4. Apparatus according to any one of claims 1 to 3, characterized in that the outlet (13) is substantially tangential to the tank sidewall (12) and arranged such that fluid leaving the tank via this outlet (13) travels opposite to the direction of flow circulation. 5. Apparatus according to any preceding claim, characterized in that both the wall/screen means (15) and the outlet (13) are arranged in the upper part of the tank (10) . 6. Apparatus according to any preceding claim, characterized in that the wall/screen means (15) extends around substantially one quarter of the tank's periphery. 7. Apparatus according to any preceding claim, characterized in that said fluid inlet (11) is arranged below said fluid outlet (13) and below said wall/screen means (15) . 8. Apparatus according to any preceding claim, characterized in that the tank is provided with a further outlet (17) . 9. Apparatus according to claim 8, characterized in that said further outlet has a pipe (18) attached thereto, the inlet (20) of which extends below the lower edge (19) of said wall/screen means (15) , said inlet (20) being approximately at the same height level as the inlet (11) . 10. Apparatus according to any preceding claim, characterized in that said apparatus is a hydrocyclone having a lower conical portion (21, 22) and a particle trap (22, 23) at its lower end. 11. Apparatus according to any preceding claim, characterized in that the outlet (13) of said tank is connected to a buffer tank of a pulping plant, and in that said buffer tank pumps fluid from said tank into a chip chute of said pulping plant without passing through a further separation section.

Inventors: GROENVOLD-HANSEN CHRISTEN, GUSTAVSSON LENNART, MOLLATT THEODOR, PERSSON KENNETH, GROENVOLD-HANSEN, CHRISTEN, GUSTAVSSON, LENNART, MOLLATT, THEODOR, PERSSON, KENNETH
Applicants: GROENVOLD HANSEN CHRISTEN, GUSTAVSSON LENNART, KVAERNER PULPING TECH, MOLLATT THEODOR, PERSSON KENNETH, GROENVOLD-HANSEN, CHRISTEN, GUSTAVSSON, LENNART, KVAERNER PULPING TECHNOLOGIES AB, MOLLATT, THEODOR, PERSSON, KENNETH
Application year: 1994
Main classification: B04C5
IPC codes: B04C5, D21D5